1. Field of the Invention
The invention relates to belt tensioning devices, and in particular to hydraulically actuated belt tensioning devices for use with the endless drive belts of the drive systems for vehicle accessories. More particularly, the invention relates to a belt tensioner operable from the low-pressure lubricating system of the vehicle engine, which maintains a predetermined constant tensioning force on the endless drive belt regardless of whether the engine is on or off, and which provides a shock-absorbing damping action on the tensioner.
2. Description of the Prior Art
There is the trend today in the automobile industry to operate the various vehicle accessories, such as the power steering pump, oil and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crankshaft. This system is referred to as a "serpentine" drive belt system. To insure optimum operating efficiency for these various accessories, it is necessary that the drive belt be maintained at a predetermined tension to assure efficient performance of the accessories as well as satisfactory service life for the belt. Due to the relatively greater length of the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch, which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension thereon regardless of the amount of belt stretch.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioner constructions, which use an elastomeric material, have the disadvantages in that the high load rate which they exert on the belt results in the rapid loss of tensioning as the belt stretches, and this load rate limits the stroke of the belt-engaged idler pulley to a shorter distance than desired. Also, sudden acceleration and deceleration of the drive belt can cause a whipping action to occur which creates a time lag before full damping is achieved.
Numerous other types of belt tensioning devices use springs for applying and maintaining the tensioning force on a belt-engaging idler pulley or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019, 2,893,255, 3,413,866, 3,483,763, 3,631,734, 3,768,324, 3,812,733, 3,924,483 and 3,965,768. Some of these various spring-actuated devices use the biasing force of the spring in combination with hydraulic-actuated members for regulating the amount of tensioning force applied to the belt, depending on whether the engine is running or shut off.
U.S. Pat. No. 2,051,488 shows a chain tensioning device in which a hydraulic cylinder is actuated when the engine is running to reduce the tensioning pressure which is exerted on the drive chain by a biasing spring. This hydraulic cylinder applies a counter force to the biasing action of the spring when the engine is running in order to decrease the tensioning force on the chain, whereupon the full biasing force of the spring is exerted on the chain when the engine is off and the hydraulic cylinder inactive. U.S. Pat. No. 3,142,193 discloses another belt tensioner using a hydraulically actuated bellcrank and belt-engaging pulley in which a spring biases the pulley into tensioning engagement with the belt until the engine is operating, whereupon the hydraulic-actuated piston imparts a greater tensioning force to the pulley through the bellcrank. When the engine stops, the piston retracts and the spring maintains a lesser tensioning force on the belt. U.S. Pat. No. 4,077,272 discloses another belt tensioner using both a spring and a hydraulic member to achieve the desired tensioning characteristics. In this device, when the engine is running, hydraulic oil from the engine lubricating system forces a piston back into its cylinder to overcome an internally mounted first spring so that only a second spring acts on the belt tensioning pulley when the engine is operating. When the engine is off, both springs acts on the belt tensioning pulley. U.S. Pat. No. 3,132,596 shows a belt tensioner using a hydrostatic snubber controlled by the power steering pump of the vehicle, whereupon the snubber is forced outwardly in direct relationship to the output pressure of the power steering pump. This mechanism does not maintain a constant pressure on the belt when the vehicle engine is both on and off, and will apply different forces in relationship to the engine speed.
Many of these devices are believed to perform satisfactorily for their intended purpose, but it is preferred that the use of springs for effecting the tensioning force on a drive belt or chain, either for applying or retracting a tensioning member therefrom, presents problems. The operating characteristics of these springs will change over the life of the spring and even in response to changes in ambient temperature. Also, as the drive belt stretches, the spring's biasing effect changes, making it difficult to maintain a constant tensioning force on the drive belt. Likewise, the springs may become rusted and corroded and break during the life of the vehicle, presenting mechanical maintenance problems for the vehicle owner.
Accordingly, it is desirable to provide a belt tensioning device which eliminates the use of springs for controlling the movement of a belt-engaging idler pulley, which device will maintain a constant predetermined tension on the belt throughout the full stroke of the pulley whether the engine is on or off or being driven at high or low speeds, and which device uses the oil of the vehicle hydraulic system for operation thereof.
Many of these problems are believed to be eliminated by the improved hydraulic belt tensioner construction shown in a copending application of Nolte V. Sproul, Ser. No. 058,362, filed July 18, 1979, which is assigned to the same Assignee as is the present application. This improved belt tensioner is operated by the relatively high pressure exerted by the hydraulic fluid of the vehicle power steering pump. However, problems could be encountered with this tensioner if used with those vehicles not having a power steering mechanism and the high fluid pressure developed thereby.
The power steering pump of many vehicles produces a hydraulic pressure in the range of from 1350 to 1450 psi, whereas the lubricating system of the vehicle generally operates at a pressure of between 30 and 55 psi. Therefore, in order to develop sufficient tensioning force on a belt-engaging idler pulley by the outward movement of a piston rod, and exceedingly large, expensive and impractical cylinder and/or bellcrank lever arrangement may be required if operated by this low fluid pressure. Thus, the need exists for an improved hydraulic belt tensioner which is operated by the relatively low pressure of the vehicle lubricating system, and which will provide the necessary tensioning force on an endless belt in a compact, efficient and inexpensive manner.
There is no known belt tensioning device of which I am aware which imparts a constant predetermined tensioning force on an endless accessory drive belt by an idler pulley which is actuated entirely by the low pressure of the hydraulic fluid from the engine lubricating system without the use of any springs or similar biasing means and which maintains this constant pressure on the belt whether the engine is on or off or operating at various speeds, and which prevents belt whip and achieves a highly efficient damping effect.